usb: gadget: f_midi: use usb_gstrings_attach

In order to add configfs support the usb_gstrings_attach must be used.

BACKPORT FROM MAINLINE KERNEL

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Change-Id: I65b34edbfa8882e8d7e82d0732fa323a2426bf3e